1. Field of the Invention
The present invention relates to a method of manufacturing a device such as a semiconductor laser device, a crystal substrate of III-V compounds of the nitride system and a crystal film of III-V compounds of the nitride system which are used in the method of manufacturing a device and a method of manufacturing a crystal of III-V compounds of the nitride system.
2. Description of the Related Art
In general, the manufacturing process of a device such as a semiconductor laser device or a light-emitting diode (LED) involves the deposition of semiconductor films or the like laid one upon another over the surface of a crystal substrate or a crystal film comprised of III-V compounds of the nitride system, e.g., gallium nitride (GaN). A desirable method to obtain a crystal substrate or a crystal film for the use in the manufacturing process is to form a bulk of a crystal of III-V compounds of the nitride system, but the manufacture of a bulk of a crystal of III-V compounds of the nitride system is difficult. Thus, it has been the practice to grow epitaxially a crystal of III-V compounds of the nitride system on a basal body made of sapphire (Al2O3).
However, the difference in the crystal structures of the basal body and the crystal formed thereon, or the small interaction between the basal body and the crystal formed thereon causes dislocations to occur from the interface. Such dislocations develop in the direction of crystal growth, piercing the crystal to reach its surface. This causes a problem that crystal substrates or crystal films thus grown have numerous defects in the surface.
To overcome the problem, Publication of Japanese Unexamined Patent Application No. Hei 10-312971 proposes a method in which a gallium nitride (GaN) base layer is formed over the surface of a sapphire (Al2O3) basal body, and a mask pattern made of silicon dioxide (SiO2) is formed over the base layer, and then a gallium nitride (GaN) crystal is grown over the base layer with the mask pattern in between. According to the method proposed, the development of dislocations is prevented by the mask pattern, resulting in a reduction in the number of dislocations piercing the crystal to reach its surface. The method, however, cannot sufficiently reduce the number of dislocations in crystal substrates or crystal films because some dislocations develop through windows of the mask pattern to pierce the crystal.
According to Extended Abstracts (The 46th Spring Meeting, (1999); The Japan Society of Applied Physics and Related Societies, page 416, disclosed is a method such that a recessed region is formed on the surface of an base layer made of GaN or the like formed on a basal body made of Al2O3 and GaN crystals are re-grown on the surface of the base layer. Since the method causes a change in the direction of the development of dislocations at the recessed region of the base layer, the number of dislocations piercing the crystal reduces to some extent. In the method, however, dislocations propagate through the portion other than the recessed region of the base layer. Thus, the method has a problem that the number of dislocations piercing the crystal cannot be reduced sufficiently.
On the other hand, according to MRS Internet J. Nitride Semicond. Res. 4S1, G3. 38 (1999) and MRS Internet J. Nitride Semicond. Res. 4S1, G4. 9 (1999), proposed are methods of preventing dislocations piercing the crystal using lateral growth. In the former method, a groove is formed in a gallium nitride (GaN) crystal as a seed crystal, and the crystal growth is re-started in the lateral direction from the walls of the groove. In the latter method, after a mask pattern is formed on the surface of a gallium nitride (GaN) crystal as a seed crystal, the gallium nitride (GaN) crystal is etched to form a groove, and the crystal growth is re-started in the lateral direction from the walls of the groove, while the mask pattern prevents the crystal growth over the surface of the seed crystal. However, with the former method, there is a possibility that dislocations will develop from the surface of the seed crystal. With the latter method, there is a possibility that other dislocations will develop above the mask pattern. Thus, both methods have a problem that defects in the surface of the crystal cannot be prevented sufficiently.
The present invention has been achieved in view of the above problems. It is an object of the invention to provide a crystal substrate of III-V compounds of the nitride system and a crystal film of III-V compounds of the nitride system, a method of manufacturing a crystal of III-V compounds of the nitride system used in the manufacture of the crystal substrate of III-V compounds of the nitride system and the crystal film of III-V compounds of the nitride system, and a method of manufacturing a device employing the method of manufacturing a crystal of III-V compounds of the nitride system.
The method of manufacturing a crystal of III-V compounds of the nitride system of the present invention comprises a step of forming a crystal layer made of III-V compounds of the nitride system on a surface of a basal body, and the crystal layer is provided with at least one inner layer made of III-V compounds of the nitride system having a different composition from the composition of the crystal layer.
According to the method of manufacturing a device of the invention, a device is manufactured by forming a device film on a surface of a crystal substrate or a crystal film. This method includes the step of growing a crystal layer made of III-V compounds of the nitride system on a surface of a basal body, and the crystal layer is provided with at least one inner layer made of III-V compounds of the nitride system having a different composition from the composition of the crystal layer.
The crystal substrate of III-V compounds of the nitride system of the present invention comprise a crystal layer made of III-V compounds of the nitride system on a surface of a basal body, and the crystal layer is provided with an inner layer made of III-V compounds having a different composition from the composition of the crystal layer.
According to a crystal film of III-V compounds of the nitride system of the present invention, a crystal layer made of III-V compounds of the nitride system is formed on a surface of a basal body, and the crystal layer is provided with an inner layer made of III-V compounds having a different composition from the composition of the crystal layer.
According to the method of manufacturing a crystal of III-V compounds of the nitride system, the method of manufacturing a device, the crystal substrate and the crystal film of the present invention, an inner layer having a different composition is formed in the crystal layer. Thus, propagation of dislocation bends by the inner layer, and the number of dislocations which propagates crystals is greatly reduced due to the interference between dislocations. Further, the inner layer is made of III-V compounds, so that further propagation of dislocations does not occur at the interface between the inner layer and the crystal layer.
Other and further objects, features and advantages of the invention will appear more fully from the following description.